Flying Flea Designs Taming the tandem wing requires analysis of the ’s stability

he first time I saw it was shortly after I arrived at Oshkosh 1976: a tail- less, glider being pushed aloft by a noisy two-stroke engine. John Moody was making the first large-scale public demonstration of his powered Icarus II. The ultralight era had begun. As a teenager with limited financial means, I thought I’d found a T way I could afford to fly. Enthusiasm for ultralights increased yearly as more people, many of whom had never been involved in aviation, Neal Willford also saw the ultralight as their only means of experiencing affordable flight. The ultralight’s popularity continued to increase until December 1983, when the infamous 20/20 episode question- ing its safety and lack of regulation dealt a near-fatal blow to the industry. The past 20 years has shown that ultralight critics were wrong, but the magic was gone, and the ultralight movement was never the same. That was not the first time in aviation history that the bubble had burst on an entire type of airplane. Seventy years ago a Frenchman named Henri Mignet set out to design a safe, easy-to-fly airplane “for the masses.” It caught on like wildfire in Europe and soon was being built in great numbers around the world. Unfortunately, a rash of fatal accidents due to a design flaw in its original configuration resulted in the flying public dropping it like a hot potato—even after the problem had been corrected. Mignet designed many more during the rest of his life, but JIM KOEPNICK they never achieved large-scale acceptance again.

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Mignet tirelessly promoted his “formula,” as he called it, through- out his life, with the idea evolving through design and technological changes through the years. A spreadsheet for estimating the stability, control, and performance of this unique style of airplane is available to download from the EAA Sport Aviation page on the EAA website at www.eaa.org.

Historical Overview Two key features of Mignet’s formula, a pivoting wing for pitch control and the tandem wing arrangement, were invent- ed prior to his involvement in aviation. Octave Chanute, who mentored the Wright brothers during their formative years in aviation, once said he believed the Wrights would succeed in their attempt to fly, but he was afraid that it would not be “in the best way.” Chanute felt the main wing should pivot for pitch control instead of using a separate control surface, as the Wrights were using. He even had a glider with a “rocking wing” built and sent to the Wrights at Kitty Hawk so they could fly it along- side their glider design for comparison. There is no record the Wrights flew that glider, but a mutual friend of Chanute and A smiling Henri the Wrights, Dr. George Spratt, continued developing the pivoting wing Mignet in his HM-14 concept as an alternative means for pitch control. Eventually his son Flying Flea. followed suit and designed a variety of control wing airplanes. Samuel Langley first used the tandem wing arrangement successfully. By flying a powered model in 1896, Langley had shown that powered flight was feasible, and he felt his contribution to aviation was complete. However, President McKinley asked Langley to get funds from the U.S.

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050-057 Flea 411.indd 3 10/12/04 11:02:30 PM Army to develop a man-carrying high-wing parasol design, was his problem with a vengeance during version. He did so, and two flight first successful airplane. In 1928 several years of experimenting and attempts of the Aerodrome “A” in he published a book and series of eventually adopted a short-coupled October and December 1903 ended magazine articles describing how tandem wing arrangement. in failure. to build it. His design became a The design was driven in part Langley abandoned his aeronau- huge success among the French, to keep the wingspan and overall tical experiments and just nine and eventually 200 were built. length down. His reason was a days later those two brothers from Though Mignet had succeeded practical one. He wanted custom- Ohio succeeded. in designing a popular airplane, ers to be able to build it in small Henri Mignet’s interest in avia- he had a problem: He couldn’t apartment rooms, where he envi- tion was piqued on the other fly. Mignet had tried and tried sioned many would be built if the side of the Atlantic by the Wright to learn, but he could not master design proved successful. brothers’ flights in France during the three-axis control system. His And successful it was. His 14th the summer of 1908. He started attempts at turns with his instruc- design (the HM-14) was completed experimenting with hang gliders tor would invariably end in a in September 1933 after a one- before World War I engulfed the spin. It was a crash in his HM-8 month building time. The tandem continent. He was a radio opera- that finally convinced him that wing design did not have aile- tor in the Signal Corps during the he needed to design an airplane rons, but relied on a large rudder war, and after the war he made a incapable of a stall/spin. and wing dihedral to accomplish living making radios. He started What Mignet lacked in flying turns. pursuing his aeronautical inter- ability, he more than made up Its biggest deviation from con- ests on the side. for in enthusiasm and persever- ventional was that the The HM-8, a conventional ance. He attacked the stall/spin front wing pivoted for pitch con- trol. Mignet’s “living wing” was similar in concept to the pivoting ONE CHORD MINIMUM wing promoted by Chanute and (MEASURED FROM 25% CHORD) Spratt 30 years earlier. FRONT WING TRAVEL Mignet reasoned that pivot- ing the wing gave the pilot more HINGE POINT (0 TO 12 DEGREES) (AHEAD OF 25% CHORD) immediate control over pitch than COSANDEY FLAP TRAVEL a separate control surface could 25% CHORD MINIMUM SEPARATION (0 TO 20 DEGREES UP) provide, regardless of whether that surface was a canard or conven- tional tail. He never referred to REAR WING INCIDENCE (6 DEGREES) his unconventional creation as an NACA 23112 AIRFOIL USED ON BOTH WINGS “aeroplane” in his book, but as a powered kite. Figure 1. Historical guidelines for the Flying Flea “Formula.” The HM-14 was further refined over a two-month period, which culminated in Mignet’s first solo flight that included turns. He flew for another 10 hours before return- ing to Paris to write a book detail- ing the new design and to mar- ket plans for others to build their ORIGINAL FLYING FLEA AIRFOIL own. His book was a huge suc- cess, and soon many Frenchmen were building their own copy of Mignet’s Le Pou du Ciel. The lit- eral English translation for this is The Sky Louse, but when his book was translated into English, it was NACA 23112 AIRFOIL USED ON LATER FLEAS dubbed The Flying Flea. Soon Fleas were being built in Figure 2. Comparison of airfoils used on Mignet Flying Fleas. Note very England and other English-speak- sharp leading edge on Mignet’s original Flea airfoil. ing countries. With hundreds

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050-057 Flea 411.indd 4 10/12/04 10:42:06 PM being built worldwide, it is not conventional designs. On Mignet’s ing. This is a useful benefit, because surprising that some were involved Fleas, this ratio was as low as 1- most Fleas have short wingspans in accidents, but between the sum- to-1, indicating that the aircraft’s and small engines. mers of 1935 and 1936 there were horizontal tail would need to be There is no free lunch, however, 11 fatalities involving the Flea. 2.5 to 3 times as large for a given and the rear-lifting wing imposes a Full-size examples were put into level of stability. drag penalty. While producing lift, wind tunnels in both France and Making an airplane’s horizontal the forward wing deflects the air England, and the testing showed tail larger increases its static sta- passing around it downward. The the design’s problem and how to bility and can allow the CG limit rear wing flies in this downwash fix it. English authorities decided to move further aft. This was the and therefore has to continually they would no longer issue flight case with Mignet’s HM-14, which “climb” in this sinking air. This permits for Fleas in the original according to his book had a rec- results in the induced drag of the configuration, and the flying pub- ommended CG of 41 percent MAC rear wing being about three times lic lost faith in the design. The Flea (based on the front wing’s chord). higher than if it were flying in was effectively grounded. His later designs had an aft CG undisturbed air. Though devastated by the turn of 50 percent MAC, and conse- The additional drag can be high of events, Mignet not only per- quently, an even larger tail. Mignet enough to offset the increase in severed with implementing the and his followers typically used the rate of climb due to the lower needed fixes, but continued to same airfoil and chord width on span loading on the forward wing. refine and promote the concept both wings to simplify construc- Analysis of a few Flea designs indi- until his death in 1965. Left unfin- tion by reducing the number of rib cates that the best increase in rate ished was his 40th design. Much more about Mignet and his designs Mignet reasoned that pivoting the wing gave can be found at www.flyingflea.org (including an online version of his the pilot more immediate control over pitch book). than a separate control surface could Technical Overview provide, regardless of whether that surface Mignet’s Flea has been described as a conventional airplane with a was a canard or conventional tail. really big horizontal tail; a tandem wing; or even a tailless, staggered biplane. Actually, all these descrip- jigs required, so he obtained more of climb occurs when the rear wing tions are more or less accurate. tail area by increasing its span. provides only about 20 percent Airplanes with “normal” tails A review of various Flea designs of the total lift. Increasing the typically have a center of gravity indicates that the tail span is usu- rear wing’s lifting load above this (CG) range from 15 percent to 30 ally 65 percent to 90 percent of the degrades the rate of climb to the percent of the wing’s mean aerody- forward wingspan. point that if it carries more than namic chord (MAC). The forward An airplane’s CG location also 35 percent, the additional induced CG limit is usually set by the ele- has a direct bearing on the horizon- drag is so high that the climb rate vator’s ability to hold the airplane’s tal tail load, which is usually down is worse than if it was carrying no nose up during landing, where- when the CG is ahead of the wing’s lift at all. as the aft limit is determined by aerodynamic center (approximate- However, there is a silver lin- the minimum acceptable stability ly located at 25 percent MAC). The ing to the higher induced drag of characteristics of the design. Flying reverse is true when the CG moves the lifting rear wing. Wings have an airplane aft of its CG limit may aft of this point. In this case the their highest induced drag when result in the airplane feeling sensi- horizontal tail acts as another lift- they are operating at high lift coef- tive to the pilot or, worse, in the ing surface, hence the reason for ficients. This is because induced airplane being unstable and lead- calling it a tandem wing design. drag is proportional to a wing’s lift ing to loss of control. The rear wing of a typical Flea coefficient squared. An airplane For a given CG location, the provides about 25 percent of the needs high lift and high drag for static stability largely depends on airplane’s total lift when flying at short field landings, so in this case the horizontal tail area and the the aft CG limit. This reduces the the higher induced drag of a rear- ratio of the tail arm length and span loading on the larger forward lifting wing is beneficial. the MAC. This ratio is typically wing and consequently improves Mignet’s Fleas have a reputation between 2.5-to-1 and 3-to-1 for the rate of climb and service ceil- of being able to do parachute-like

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050-057 Flea 411.indd 5 10/12/04 10:42:42 PM Mignet instead used a pivoting for- ward wing because it gave him the direct and immediate control he TRIM CONDITION preferred. Mignet correctly under- stood that a pivoting wing should require a pull force on the control stick to slow the airplane down. He accomplished this by locating the wing’s pivot point ahead of the wing’s aerodynamic center. Figure 1 shows the basic formu- la Mignet and his followers used over the years. This arrangement requires that the pilot continu- HIGH WING DESIGN ally pull back on the stick, which LOW WING DESIGN required about 7 pounds of force on the HM-14, according to Mignet. This pull force is proportional to the load factor experienced by the Lift Coeffi cient CL airplane, so for the HM-14 the pull force would increase from 7 to 14 Figure 3. Pitching moment comparison for a low-wing and high-wing air- pounds when going from 1g to 2g plane designed for the same stability at a lift coefficient of 0.2. flight conditions. The Flea designer can tailor this pull force by careful selection of the hinge location and control stick length, but it probably should not be less than 5 pounds, which is the minimum stick force per Unstable Region g the FAA allows for certificated airplanes. Having to continuously pull back on the stick is not desir- able for most pilots, so many Fleas are either equipped with an adjust- Max Speed Trim able spring attached to the control Condition stick to pull it back or trim tabs on the forward wing to relieve the Wind Tunnel Data (No Prop), 40% MAC stick force. Calculated Wind Tunnel Conditions Mignet also realized that using a Full Power Calculated, 41% MAC conventional airfoil on the forward Idle Power Calculated, 41% MAC wing would result in a pull force on the control stick that increased rapidly at higher airspeeds. This is Forward Wing Lift Coeffi cient, CL because most airfoils have a nega- tive pitching moment that tries to Figure 4. Mignet HM-14 Pitching Moment Curves rotate the leading edge down and the trailing edge up. descents. Their benign stall char- take advantage of its ability to do Using a stable airfoil—one that acteristics and the rear wing’s addi- parachute descents. has a low or positive pitching tional induced drag helps make Different means have been used moment—greatly reduces or elimi- this possible. One of Mignet’s fol- over the years to provide pitch nates this additional stick force. lowers, Louis Cosandey, added a control on tandem wing designs. Figure 2 shows the airfoil Mignet narrow upward-deflecting flap to Some have used elevators on the developed for the HM-14. It had the rear wing of the Fleas he built. rear wing. The the desired slightly positive pitch- This flap allows the Flea to fly at and its two-seat derivatives had ing moment, but as we will see a higher angle of attack to better an on the forward wing. shortly, it likely contributed to the

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050-057 Flea 411.indd 6 10/12/04 10:43:04 PM HM-14’s problems. location affects stability as well. shows the pitching moment curves Most Fleas designed after the The impact of vertical CG is a for a high- and low-wing airplane HM-14 used either the NACA both a function of the forward designed to have the same level of 23012 or 23112 airfoil. The 23112 wing’s lift coefficient and the verti- stability at a lift coefficient of 0.2. is basically a 23012 airfoil with cal CG’s location with respect to Trimming the airplanes for any some reflex added to the camber it. When the CG is below the wing other condition would move these line to give it a slightly positive (like on a high-wing airplane), the curves up or down until the curves pitching moment. A zero or posi- effect is stabilizing, but is desta- crossed the zero pitching moment tive pitching moment airfoil is bilizing for a low-wing airplane. line at the new lift coefficient. also desirable on the rear wing, This can be seen by the shape of If we draw a line tangent to these and as mentioned earlier, Fleas the two curves in Figure 3, which two curves at the trim condition often used the same airfoil and chord width on both wings. This is not mandatory, though, since the rear wing never experiences a high maximum lift coefficient. A symmetrical airfoil section could be used instead. Mignet did not use ailerons because he wanted to reduce the chance of entering a spin. Roll control was obtained by using a large rudder and excessive wing dihedral. When the rudder was deflected, the airplane would yaw and the excessive dihedral com- bined with its short wingspan induced a roll in the desired direc- tion. Mignet believed that the air- plane should be completely con- trolled by the stick, and therefore did not use foot pedals for the rudder. The major drawback of this two- axis control system is the inability to handle any significant cross- wind on takeoff or landing. This was not much of a problem in Mignet’s day because most pilots flew from large pastures instead of from narrow runways. Modern day Flea fliers either put up with this limitation, or equip their ships with roll spoilers or ailerons on the rear wing.

Trouble Unmasked So how could a design that was so easy to fly hide a problem that killed some of its pilots? There were several contributing factors. The first was the impact that the vertical CG location has on the airplane’s stability. An airplane’s horizontal CG location has the big- gest influence, but the vertical CG

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050-057 Flea 411.indd 7 10/12/04 10:43:56 PM The HM-18, Mignet’s 1936 design that incor- porated the lessons learned from the HM-14. Mignet’s subsequent designs had this same general look.

shown, they would both be sloping “heavier” in pitch when operated eral pitching moment curves for down and to the right. This is good, at higher lift coefficients (like those it, with the data points from the as the downward slope indicates a experienced when flying in the pat- British wind tunnel test shown stable airplane, and the steeper the tern). The low-wing design would along with a calculated curve using slope, the greater the stability. A be the opposite, and feel “lighter” the spreadsheet for comparison. horizontal tangent line indicates in pitch flying under the same con- The pitching moment curve shows a neutrally stable airplane, and a ditions. Low-wing airplane design- the characteristic shape of a high- tangent line with an upward slope ers need to keep this in mind when wing airplane. It also indicates that means an unstable airplane. sizing the horizontal tail to ensure the design was neutrally stable at Note how the high wing’s curve that their design is stable through- a coefficient of lift (CL) of 0.3 and gets steeper for higher lift coef- out the whole flight regime, not was unstable below that. ficients and the low wing’s curve just at cruise. The blue curve shows the cal- gets shallower. This indicates that The HM-14 behaved as a high- culated HM-14 stability at the CG a high wing airplane’s stability wing airplane from a stability location shown in Mignet’s book increases, and would therefore feel standpoint. Figure 4 shows sev- and at full power. Drawing a line

The HM-1100 CORDOUAN is the most recent design from the Mignet Aircraft Co. A true three-axis airplane, it has ailerons on the rear wing.

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050-057 Flea 411.indd 8 10/12/04 10:45:53 PM tangent to the max speed trim Mignet succeeded in develop- condition would show that it was ing a spin-proof, easy-to-fly air- References: slightly stable. At any speed slower plane. Its ability to do parachute “The Control Wing Aircraft,” (indicated by a higher CL), the style descents was a bonus fea- Spratt, George, EAA Sport Aviation, airplane’s stability increased. ture. The Flea configuration has June 1974. If the airplane were put in a unique advantages that are still The Flying Flea, Mignet, Henri, dive, the stability would become worth considering today. Had the Sampson & Low, 1935. neutral or negative. Pulling back original Flea’s flaw been uncov- Henri Mignet and His Flying on the power would have made ered and fixed before anyone got Fleas, Ellis, Ken & Jones, Geoff, matters worse, as the high thrust hurt, we might all be flying Fleas Foulis, 1990. line provided a stabilizing effect. today. This can be seen by the green curve, where a tangent line drawn on the curve at any lift coefficient Don’t Guess! Fly Safe with Angle-of-Attack

lower than the maximum speed Voice Warnings point would indicate that the air- Accurate and Affordable plane was unstable. The pilot who No Ugly Probes or Moving Parts Oshkosh Hardware True full range AOA Instrument didn’t understand what was hap- Award Lightweight and Easy to Install pening could quickly lose control Best Landing Gear Warning Option Patents 6,271,769 B1 & Pending in this situation. The HM-14 pilots who lost con- Call Now! (952) 474-4154 trol entered a dive from which Proprietary Software Systems they couldn’t recover. The wind 950 Iris Circle tunnel tests showed that the air- Professional Excelsior MN 55331 plane in its original configura- www.angle-of-attack.com tion did not have sufficient pitch Sport authority to recover from a dive if the angle of attack went below -15 degrees. The logical solution was to increase the travel of the forward wing. This was done, but it did not totally fix the problem. The sharp leading edge airfoil used on the HM-14 had a poor max- imum lift coefficient that would not let the forward wing generate enough lift to pull it out of a dive. All subsequent Flea designs used the airfoils mentioned earlier to avoid this problem. The last flaw of the original configuration was that it used a single cable to control the wing. In normal flight, this would be okay, as the wing lift would result in tension on the cable. However, once in a dive, the Flea could get to an angle of attack where the lift on the forward wing was zero or negative, and the cable would go slack. The unfortunate pilot then had no means to control the wing when it was needed the most. All subsequent Fleas used a rigid tube to control the wing. After much trial and error,

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